The current North American Numbering Plan (NANP) is based on a 10-digit telephone number format that includes a three-digit area code (also called a Number Plan Area (NPA)), a three-digit central office prefix code, and a four-digit subscriber line number. The current NANP format can be represented as:                NXX-NXX-XXXX,where N can have a value between 2 and 9, and X can have a value between 0 and 9.        
While the current NANP plan has worked well for past several decades, it is slowly approaching its capacity. The reasons for this are many. For example, back in the 1970's or even the 1980's, each family household was typically associated with a single telephone number. Today, it is not uncommon for each family member to have their own telephone number. For example, a typical household may have two or more telephone numbers associated with their house and one or more members of the household may have their own cell phone, each having a unique telephone number. This increased demand for telephone numbers has, among other factors, caused the existing 10-digit NANP to move toward its maximum capacity.
As the life cycle of the North American Numbering Plan (NANP) slowly approaches the need for serious expansion, several clever plans such as overlays, realignments, and splits, have been implemented to extend the life of the current 10-digit NANP. Long term planning by the Industry Numbering Committee (INC) has been underway to find the least intrusive and least confusing way to expand the dial plan when needed while sacrificing minimal integrity of the existing plan that everyone has grown to expect and rely on personally as well as economically.
Some proposed NANP expansion formats include increasing the number of digits in the area code (e.g., (N)XXX-XXX-XXXX or N(X)XX-XXX-XXXX), adding a National Destination Code (NDC) (a 1-digit code other than 0 or 1) to the beginning of the current 10-digit NANP (e.g., NDC+NXX-XXX-XXXX), or adding a 2-digit Steering Code (SC) to the beginning of the current 10-digit NANP (e.g., SC (2-digit)+NXX-XXX-XXXX). These proposed NANP expansion formats have a number of well-documented draw backs, such as transition period problems, dialing problems or conflicts, adverse user impact, or regulatory issues.